The present invention relates to circuit substrates having a passive element such as a capacitance element, a resistance element, an inductance element, and the like, thereon or therein, or an electron device having such a circuit substrate, or a fabrication process of such a circuit substrate. More particularly, the present invention relates to a circuit substrate having a base substrate or insulation layer formed of a resin material. Further, the present invention relates to circuit substrates, passive components, electron devices and the fabrication process of a circuit substrate suitable for use in a high frequency circuit. Particularly, the present invention relates to a circuit substrate including therein an interlayer insulation film having a low dielectric loss at high frequencies and simultaneously a conductor layer of low resistance. Furthermore, the present invention relates to film-like bodies formed by spraying a fine particle material to the substrate at high speed, the formation process thereof, and structures including such a film-like body. Particularly, the present invention relates to a strong and dense film-like body formed according to such a process.
Aiming the ubiquitous society, downsizing, miniaturization and performance improvement of electronic apparatuses are going on rapidly in the field of mobile devices, and the like, including personal computers, cellular phones, Bluetooth (trade mark) devices, and the like. To promote the miniaturization of such electronic apparatuses further, there is a need of higher density packaging technology and higher integration technology of high frequency circuits. In order to achieve this goal, various circuit substrates are proposed that incorporate therein various passive elements, such as capacitors, resistance elements, inductors, antennas, filter, and the like.
In the wireless information telecommunication technology for mobile devices such as cellular phones, Bluetooth (trade mark) devices, and the like, there is a demand of transmitting large amount of signals such as voice signals, image signals, data, and the like, with very higher speed, and thus, adaptation of the electron components to the operation at such high frequency region is progressing rapidly in the art of mobile devices, in addition to the downsizing and expansion of function of the mobile device itself. Under this situation, there comes up a desire to realize a substrate including therein passive elements integrated with high frequency circuits in the form of a module, for achieving the device downsizing and simultaneously the adaptation to high frequency applications
In the packaging substrates and packages, or in the memory semiconductor devices or logic semiconductor devices, or in the discrete electronic components for packaging for use in the information processing-related electronic apparatuses such as personal computers, business computers, cellular phones, PDAs, and the like, or in the communication-related electronic apparatuses or the control apparatuses such as the one used for semiconductor production, various dielectric ceramics are used in the form of film or a bulk material for implementing various active functions such as memory or computational functions or passive functions. The example of such a passive element includes antennas, filters, capacitors, and the like. These devices or components are generally not formed of a single ceramic material but usually forms a composite body, a multilayer body, or integrated body of various materials such as inorganic materials (metal or semiconductor) or organic materials.
The circuit substrates that incorporate therein the passive elements developed until today can be classified generally into three types: (1) the substrates in which the passive elements are formed by a thin film process by using a silicon substrate, and the like; (2) the substrates in which the passive elements are formed by using a ceramic substrate; and (3) the substrates formed by using a resin-base printed circuit board.
In the case the passive element is formed by a thin film process technology according to the type (1), a multilayer structure is obtained by repeatedly depositing, on a flat surface of a silicon substrate or an alloy substrate, interconnection layers by a sputtering process and insulation layers by a coating process of a resin layer such as polyimide. In this case, the passive element such as the dielectric film of a capacitor or a resistance pattern of a resistance element is formed by a thin oxide film. For the dielectric film, oxide ceramic materials such as BT (BaTiO3), BST (BaSrTiO3), and the like, formed by a sputtering process, sol gel process or a CVD process, are commonly used. For example, reference should be made to Japanese Laid-Open Patent Application 2001-250885 official gazette.
In the case of forming the passive element by using a ceramic substrate according to the type (2) noted above, a multilayer structure is formed on a ceramic substrate by repeatedly printing, drying and baking a paste of a conductor film, a dielectric film, a resistance film, or an insulation film. Because the baking process is carried out at the temperature of 1000° C. or more, the dielectric film of this type can more or less realize the ideal dielectric characteristic of a bulk material.
In the case of forming the passive element by using a resin printed circuit board according to the type (3), on the other hand, FR4 (a glass epoxy material) is used for the base substrate and a Cu film formed by a plating process is used for the electric conduction layer. Further, for the insulation layer, an epoxy resin sheet or an epoxy resin varnish (heat resistant temperature: 250° C.) is used.
Thereby, via-holes are formed in the insulation layer by a laser drilling process, and the via-holes thus formed are filled by a metal plating process or by a powder material to form via-plugs. The dielectric film of the capacitor is incorporated into the circuit substrate, by forming a dielectric material by mixing a ceramic powder having dielectricity with a binder resin and by forming a sheet or pattern of the dielectric material from the mixture.
In another case of using the resin printed circuit board according to the type (3), a passive component is mounted on the insulation layer provided on the base substrate and the structure thus formed is covered with an insulation sheet such that the passive element is incorporated in the component level.
However, in the circuit substrate that uses the thin film process of the above type (1), there is a need of conducting a substrate heating process or post annealing process at the temperature of 400° C. in the lowest, in any of the cases in which the dielectric film is formed by the sputtering process or sol gel process or CVD process. In the case of using polyimide as the insulation layer, on the other hand, it is necessary to reduce the temperature of the substrate heating down to 350° C. or lower for securing the reliability of the circuit substrate in view of the fact that the heat resistant temperature of polyimide is only about 400° C. However, in the case that the substrate heating process or post annealing process is conducted below the temperature of 350° C., crystallization of the dielectric film does not proceed sufficiently and there arises a problem in that the dielectric characteristics of the dielectric film is substantially inferior as compared to the case of a bulk material.
For example, while a BT film has a specific dielectric constant of 1500-3000 in the bulk state, the forgoing approach provides the dielectric constant of only about 200. Thereby, there arises a problem in that the electrostatic capacitance of the capacitor is reduced.
Further, there is proposed an approach of using a highly refractory alloy substrate or a ceramic substrate for the base substrate and provide the dielectric film only once on such a base substrate, followed by applying a heat treatment at the high temperature, and then laminate an insulation layer of a resin material. However, such an approach can form only one layer of the dielectric film, and there arises a problem in that the electrostatic capacitance formed in the circuit substrate is limited.
With regard to the circuit substrate that forms the passive elements by using a ceramic substrates according to the type (2), there arise the problem of high cost due to the need of using high temperature over 1000° C. at the time of the baking process. Further, there arises the problem of poor yield caused by possible short circuit, electrical disconnection, deformation, and the like, which in turn may be caused at the time of the baking process as a result of large difference of thermal expansion coefficients between the insulation film, the electric conduction film and the passive components.
In the case of forming the passive element by using the resin printed circuit board the type (3), on the other hand, there arises a problem, associated with the fact that the dielectric constant of the dielectric film is lower than about 100, in that the electrostatic capacitance capable to be realized in the circuit substrate is limited.
Further, with regard to the alternative approach of the type (3) of forming the passive elements by using a resin printed circuit board, there arise the problems in that there is a need of forming a cavity for introducing the passive element and mounting the passive element into such a cavity, while such a process is complex and it is difficult to secure high reliability for the connections between the passive elements and the wiring. In addition, there arises a problem in that repairing, such as replacement of defective passive elements, is difficult. Further, there arise additional problems in that: the structure becomes complex in view of the necessity of forming a cavity in the insulation film; it is difficult to secure planarization of the layer in the upper part of the circuit substrate; and that the layer number of lamination is limited.
Meanwhile, in a high frequency circuit, the loss is represented by a sum of conductor loss and dielectric loss (dielectric tangent), and the effect of the dielectric loss increases with increase of the frequency. Thus, the dielectric material for use in a high frequency circuit is required to have a low dielectric loss.
However, because of the fact that the interlayer insulation films of the above types (1) and (3) are formed of a resin material having a large dielectric loss, there arises a problem in that the loss increases sharply at high frequencies. For example, a polyimide resin has a dielectric loss of 0.004 at 2 GHz while an epoxy resin has a dielectric loss of 0.0125 at 2 GHz.
On the other hand, the interlayer insulation film of the type (2) is formed of a low dielectric ceramic material, and thus, the use thereof is thought promising for high frequency applications.
At present, a method that uses a LTCC process (low temperature baking ceramics process) is used for the production of ceramic substrates for high frequency applications. In the LTCC process, a ceramic material of low baking temperature containing a glass as a baking additive and a conductor paste containing a metal powder of low electrical resistance are printed respectively as the interlayer insulation film and the conductor pattern, and the interlayer insulation film and the conductor thus printed are baked simultaneously. For the metal powder of the conductor paste, Ag, Cu, Au, and the like, of low electrical resistance are used commonly.